AMMAN (Reuters) - Islamic State prisoners on Sunday seized control of the ground floor of a major prison in northeastern Syria under the control of the U.S.-backed Syrian Democratic Forces (SDF), with some of the militants managing to escape, an SDF spokesman said

The SDF, which is spearheaded by the Kurdish YPG militia, mounted an operation to capture those who fled as security forces sent reinforcements to crush the mutiny in Hasaka prison, spokesman Mustafa Bali said in a tweet.

"The situation is tense inside the prison currently and we sent anti-terror forces and additional troops to control the situation," Bali said.

Earlier, Syrian state television said 12 militants had fled from the prison towards the southern outskirts of Hasaka.

Arab tribal figures in touch with residents in the area said U.S. coalition planes were seen flying overhead in the vicinity of the prison facility following the incident.

There were unconfirmed reports that several inmates had been killed in the latest of several attempts to flee from SDF prisons, according to residents.

It was not clear how many inmates were in the prison, one of several where the SDF has kept thousands of detainees, many of whose relatives say are young children and others arrested on flimsy charges or for disobeying the SDF's policy of forcible conscription.

U.S.-based Human Rights Watch says the SDF holds about 12,000 men and boys suspected of Islamic State affiliation, including 2,000 to 4,000 foreigners from almost 50 countries.

The inmates are held in overcrowded prisons where, according to Human Rights Watch, conditions are inhumane in many cases, a description that echoed concerns by other rights groups.

The Kurdish-led forces also hold about 100,000 Syrian and foreign women and children who are family members of militant suspects in squalid camps across the areas they control.

Some Arabs, who form a majority of the inhabitants of the areas under the control of Syrian Kurdish YPG militia, accuse the Kurds of discrimination, an allegation rejected by Kurdish officials.

With crucial U.S. air and ground support, the SDF defeated jihadists across north and east Syria. lslamic State has resorted to guerrilla attacks since losing its last significant piece of territory in Syria last year.